There is considerable interest in using rare earth doped optical fiber amplifiers to amplify weak optical signals for both local and trunk optical telecommunications networks. The rare earth doped optical amplifying fibers are found to have low cost, exhibit low-noise, display a relatively large bandwidth which is not polarization dependent, present substantially reduced cross talk problems and provide low insertion losses at the relevant operating wavelengths which are used in optical communications. Rare earth doped optical fiber amplifiers can be coupled end-to-end to a transmission fiber, and coupled, through a directional coupler, to a laser diode pump. The directional coupler is designed to have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength so that maximum pump energy is coupled to the amplifier with minimal signal loss. When the amplifying medium is excited by the pump laser, signal light traversing the amplifier experiences gain. The pump energy can be made to propagate either co-directionally or contra-directionally relative to the signal energy.
In optical fiber systems that use optical amplifiers as repeaters, the laser diode that supplies the pump power to each fiber amplifier is the most expensive and the only active component. As such, these laser pumps can be considered to be the most likely component in the system to age and eventually fail.
In systems where high levels of system availability and reliability are a requirement, such as, for example, in undersea systems, redundant laser pumps for each amplifier are required as a backup in the event that a laser pump fails.
The present invention is directed toward a new, improved and more economical arrangement for providing a redundant laser pump for optical amplifiers.